Oil containing liquids and, conversely, liquids containing oil are used for many purposes.
Degreasing is used for removing oil, fat, organic substances and the like, as well as non-organic particles from goods and other matter. As used herein the term "degreasing" is thus not limited to removing contaminating grease, but it includes any fatty, grease, or functionally similar contaminating matter. The most important chemical agents for industrial degreasing and purification are chlorinated solvents such as methylene-, tri- and per-chloroethylene and the like, as well as water-soluble alkaline degreasing agents.
In Sweden at least 10,000 companies carry out degreasing. The costs for degreasing amount to about 1500 million SEK per year. The investments made in degreasing apparatuses correspond to an estimated 1000 million Swedish crowns. Only as regards destruction costs these were about 1750 SEK/cubic meter.
Water-based degreasing can provide increased purity compared to solvent-based degreasing since it, when carried out correctly, can remove both oil, fat, particles and salts.
Because the nature of most of the relevant metals is that they have a negative surface charge, alkaline degreasing is a most common and most efficient cleaning method. Neutral degreasing can be suitable when the goods to be cleaned only consist of light and recently greased material. Acid degreasing is used to a considerably less extent and is primarily applicable when it is desirable to simultaneously get a certain de-oxidation (for example metals from the type aluminum and stainless steel).
Because of inner and outer environmental problems when using chlorinated solvents, conventional alkaline degreasing is presently the quickest. The alkaline degreasing agents are built-up of an alkaline substance (e.g. sodium hydroxide or sodium carbonate), complex forming agents (such as tripolyphosphate), and surfactants, usually non-ionic tensides.
In water-based degreasing the compounds are emulgated and fats and oils which can be saponified are converted into water-soluble soapes. Mechanical energy has to be added for the dispersion to be obtained efficiently and within reasonable time. Efficient degreasing also requires a high temperature, which is above the melting point of the impurity.
After a certain time of service the cleaning properties of the degreasing bath deteriorate. This is most often caused by contamination by oil and particles. Other common contaminants in the washing liquid system are hydraulic oil, mineral oil, emulsified oil, vegetable oil/fats, wax, graphite, abrasives etc. The service length of the degreasing baths vary from days to months depending on the application. A prolongation of the service length can many times be achieved by separating particles and free oil by means of a physical method such as an oil skimmer, hydrocyclone, centrifugal separator, lamell separation, paper filter or membrane filter techniques. The last-mentioned technique is primarily used when a great part of the oil is emulsified into the degreasing bath. The materials which are recovered by membrane filtration are primarily alkali, phosphate and water while the main part of the surfactants will be removed by the filter.
A prolonged operation time does not only mean reduced disposal costs, but also reduced fixed costs, higher quality and better working environment.